Mechanisms associated with the depigmentation of brown eggshells: a review

Eggshell color is an important shell quality trait that influences consumer preference. It is also of particular importance with respect to sexual signaling and the physiological and mechanical properties of shell pigment. Pigments include protoporphyrin IX, biliverdin, and traces of biliverdin zinc chelates, with brown eggs being notably rich in protoporphyrin IX, the synthesis of which has a marked effect on the intensity of brown eggshell color. This pigment is initially synthesized in the eggshell gland within the oviduct of laying hens and is subsequently deposited throughout the cuticular and calcareous layers of brown eggshell. In this review, we describe the factors affecting brown eggshell color and potential targets for the regulation of pigment synthesis. Protoporphyrin IX synthesis might be compromised by synthetase-mediated pigment synthesis, the redox status of the female birds, and regulation of the nuclear transcription factors associated with δ-aminolevulinic acid synthetase1. We believe that this review will provide a valuable reference for those engaged in studying eggshell depigmentation.